Digital watermarking is a process for modifying physical or electronic media to embed a hidden machine-readable code into the media. The media may be modified such that the embedded code is imperceptible or nearly imperceptible to the user, yet may be detected through an automated detection process. Most commonly, digital watermarking is applied to media signals such as images, audio signals, and video signals. However, it may also be applied to other types of media objects, including documents (e.g., through line, word or character shifting), software, multi-dimensional graphics models, and surface textures of objects.
Digital watermarking systems typically have two primary components: an encoder that embeds the watermark in a host media signal, and a decoder that detects and reads the embedded watermark from a signal suspected of containing a watermark (a suspect signal). The encoder embeds a watermark by subtly altering the host media signal. The reading component analyzes a suspect signal to detect whether a watermark is present. In applications where the watermark encodes information, the reader extracts this information from the detected watermark.
Several particular watermarking techniques have been developed. The reader is presumed to be familiar with the literature in this field. Particular techniques for embedding and detecting imperceptible watermarks in media signals are detailed in the assignee's U.S. Pat. Nos. 6,614,914 and 6,122,403, which are hereby incorporated by reference.
One challenge for watermarking visual media is maximizing the robustness, readability, and/or auxiliary information capacity of a digital watermark relative to a desired perceptual quality level. The digital watermark modifies the host image to embed auxiliary data, yet makes these changes using the assistance of human visual system modeling such that the changes are substantially imperceptible. While many have discussed perceptual image models that address this issue, several challenges remain, including effectively extending models beyond just luminance to address human perceptual models for colors, and addressing the impact of printing to the perceptual model.
This disclosure describes a method for digitally watermarking visual media taking into account a model of the rendering device, such as the printer or video display, to account for the effect of the rendering device on visibility of the digital watermark. In one implementation, a reference image and watermarked image are passed through a model of the rendering device and then to a perceptual quality metric to compute a visibility metric for the watermarked image relative to the reference. The digital watermarking process repeats this evaluation iteratively, while adjusting the embedding of the digital watermark to achieve a desired visual quality.
In the case where the rendering device is a printer, the model of the output device takes into account such effects as the blurring effect, the tonal compression effect, and/or the dot gain effect of the printing process for a particular printer and substrate on which the image is printed. A similar approach can be applied to display devices, such as video monitors, video projectors, high definition video monitors, Plasma displays, etc. to take into account a model of the rendering device. As such, the method applies to still images and video content.
The technique can also be applied to watermarked audio, where the rendering device produces audio output and the system includes a model of the rendering device as well as an audio quality metric.
In our implementation, we use the Watson metric as a visual quality metric, but other metrics may be used as well.
Further features will become apparent with reference to the following detailed description and accompanying drawings.